This invention relates to a hydrocarbon dispensing line pressure transducer or pressure probe for sensing a leak in the dispensing line. More specifically, this invention is an improvement on U.S. Pat. No. 4,835,717 that is hereby incorporated by reference.
A typical use for a hydrocarbon product dispensing line pressure probe is in a vehicular service station that dispenses fuel. In a vehicular service station, it is common for fuel to be stored in large underground storage tanks and pumped through a product dispensing line to a dispenser. The dispenser typically has a switch that is turned on by a customer to activate a pump for pumping fuel through the product line into a vehicle. When the customer has completed dispensing fuel, the dispensing switch is turned off which also turns off the pump. A check valve in the product dispensing line prevents fuel from draining out of the product dispensing line when the pump is turned off and maintains the product under pressure in the product dispensing line. A pressure relief valve built into the pump reduces product line pressure to about 11-15 pounds per square inch PSI (0,759-1.034 Bar) after the dispensing switch is turned off.
The product dispensing line also has a line pressure probe installed in a "T" connection in the product dispensing line for sensing product line pressure. In the past, line pressure probes have been designed to detect large scale leaks on the order of three (3) or more gallons (7.78 or more liters) per hour. When detecting large scale leaks, thermally induced pressure variations are less significant.
Recently, the Environmental Protection Agency ( E. P. A. ) has required that small scale leaks measured in fractions of a gallon (liter) per hour must be detectable in product dispensing lines. The Environmental Protection Agency ( E. P. A. ) requires either an annual line tightness test with a required detection rate of "0.1 gallon per hour [0.379 liters] leak rate at one and one-half times the operating pressure," 40 C. F. R. .sctn. 280.44 (b) (1988) , or a monthly line tightness test with a required detection rate of "0.2 gallon per hour [0.757 liters] leak rate or a release of 150 gallons [567.81 liters] within a month with a probability of detection of 0.95 and a probability of false alarm of 0.05." 40 C. F. R. .sctn. 280.43 (h) (l) (1988).
With the E. P. A. mandated line tightness test standards, thermally induced pressure variations became significant, and a there was a need to distinguish between thermally induced pressure changes and an actual product dispensing line leak. Some previous hydrocarbon leak detection systems, such as that described in U.S. Pat. No. 4,835,717, have employed a resistive thermal device, such as a thermistor, to sense product temperature in an effort to compensate for thermally induced pressure changes. Use of a resistive thermal device can create inaccuracies because product temperature is only measured at one location, and product temperature can vary many degrees over the length of a product line.
Previous hydrocarbon leak detection systems have determined whether thermal contraction was present by measuring pressure at various times depending upon product line pressure.
Previous hydrocarbon leak detection systems test for thermal contraction to distinguish a pressure change caused by a leak from a pressure change caused by thermal contraction. If thermal contraction is tested, a leak which varies with thermal conditions could appear as thermal contraction. For instance, product line elbows are typically made from a thicker material than the product line. If the liquid product is at a different thermal differential from the product line, the product line would undergo thermal contraction or expansion more quickly than the product line elbow because of the greater mass of the product line elbow. If there was a leak in the fitting between the product line elbow and the product line, the leak rate could vary with temperature changes and appear as thermal contraction.
What is needed is an intelligent pressure probe with thermal compensation that measures pressure at defined time intervals by changing a threshold pressure for determining failure based upon thermally induced product contraction.